The Gamma distribution of order a > 0 is defined by:
probability(x) = {1 / Gamma(a) scale^a} x^{a-1} exp{-x/scale}
with x > 0. The value scale is called the scale parameter.
If X and Y are independent gamma-distributed random
variables of order a1 and a2 with the same scale parameter scale,
then X+Y has gamma distribution of order a1+a2.